The corefx repo contains the library implementation (called "CoreFX") for .NET
Core. It includes System.Collections, System.IO, System.Xml and many other
components. You can see more information in Documentation.

.NET Core is a modular implementation of .NET that can be used as the base
stack for a wide variety of scenarios, today scaling from console utilities to
web apps in the cloud.

Runtime-specific library code - namely mscorlib - lives in the CoreCLR repo. It
needs to be built and versioned in tandem with the runtime. The rest of CoreFX
is agnostic of runtime-implementation and can be run on any compatible .NET
runtime. These characteristics were the primary motivation for the 2-repo
structure.